This invention relates generally to an energy conversion system and, more specifically, to an energy conversion system which is capable of converting three-phase high frequency energy to single phase, relatively low frequency energy as, for example, 60 cycle energy, and is an improvement on the system shown in copending patent application to Young et al, Ser. No. 542,249, filed Jan. 22, 1975, the disclosure of which is herein incorporated by reference.
With the increased use of electrical energy in remote areas, a great demand has arisen for portable or emergency energy sources of electric power. Typically, these sources of electrical power are utilized to drive appliances and the like which are designed to efficiently operate at 60 hertz alternating power. Accordingly, there is a need for a relatively low cost, portable source of alternating current, 60 hertz energy.
A common means of generating alternating current energy is by a generator driven by a prime mover. It has long been known that the size, and therefore the expense, of building a generator to produce a preselected power output decreases as the frequency increases. One problem is that the standard in the industry has become 60 hertz in the United States, and in many other countries of the world, and hence the use of high frequency generators is not practical for many applications.
With the present invention a means is provided of utilizing a high frequency generator incorporating highly developed prime mover and generator components presently available on the market to produce a low frequency output.
This is accomplished by utilizing a conventionally available prime mover which, for example, can via belt drive etc., have a 9,600 rpm output to drive a high speed, three-phase, multipole, for example 12 to 16 poles, alternator of conventional design. Alternators of this type have been employed in electrical systems in the automotive industry for a number of years.
In one form of the invention, the prime mover is operated to rotate the field windings of a twelve pole alternator at 9,600 rpm which is adapted to generate a 960 hertz output from a polyphase wye connected armature winding. The output of the three-phase armature winding is approximately equal to the desired peak voltage of the sinusoid waveform that the system reconstructs at the output. This output voltage is approximately 160 volts peak. For purposes of this specification, the use of the term hertz is intended to include a repetitive waveform, whether alternating or unipolar.
The output of the three-phase 960 hertz alternator is fed to a full wave rectifier bridge through a resolver device which is utilized to control the output from the bridge in synchronism with the operation of alternator via the prime mover to provide a modulated wave approximating a sinusoidal shape.
Subsequently, the switching of the power output from the resolver is accomplished by the inverter device and occurs twice per output cycle. The inverter is devised and synchronized to perform its switching function at a point when the output voltage is zero. Where the load is resistive the current and voltage will be in phase and switching at zero volts will provide switching at zero current as well. However, where inductive or low power factor loads are encountered the current at zero volts could be substantial and could result in arcing at the switches of the resolver and/or inverter. This problem is solved by an arc suppressor circuit which is actuated prior to opening of the load circuit to provide a shunt path around the switch which permits switching without damaging arcing.
The prime mover is, in one form of the invention, adapted to drive alternator 12 at 9,600 rpm. The resolver and inverter devices are interconnected and are driven in turn by a gear reduction system whereby the resolver and inverter devices rotate at a speed which is one-half the speed of the prime mover or 3,600 rpm. In this way, the resolver provides a modulated pulsating direct voltage output at 120 hertz; the inverter operates to invert alternate cycles from the resolver to provide an output therefrom to the load at 60 hertz.
As will be seen from a further description of the system, the output from the inverter is not, in the voltage sense, a sinusoidal waveform. Rather, the output is approximately at a constant voltage, for example 160 volts. However, the average output generated at the output terminals of the inverter varies as a 60 hertz sinusoidal wave. In the preferred embodiment, this is accomplished by the resolver in first switching in the first phase from the alternator (as rectified) to provide a short duration pulse of 160 volt amplitude. Subsequently, a second phase (as rectified) is switched in to add to the output energy from the first phase. Subsequently, the third phase from the alternator (as rectified) is switched in whereby all three phases are conducting and energy is flowing from the output inverter substantially continuously.
The situation then reverses whereby the first phase is switched out and the second and third phases provide approximately two-thirds of the full load average current, and next the second phase is out leaving only the third phase conductive. At the end of the cycle, all three phases are switched out and the inverter switches the subsequent waveform to the opposite polarity and presents this inverter waveform at its output. It is at the switching off of the third phase that the arc suppressor circuit noted above is operative. From the foregoing, it is seen that each phase provides approximately an equal amount of output current relative to the other phases. Thus, no one phase provides substantially more energy than any other phase in the system.
In addition to the above the present invention provides means to protect the alternator from improper loads by sensing excessive voltage rises at the output and shutting down the alternator in response.
Further, the system provides means to protect the alternator from load faults and to shut the alternator down in response thereto.
Accordingly, it is one object of the present invention to provide an improved prime mover and generator set for generating alternating current low frequency energy.
It is another object of the present invention to provide an improved prime mover and generator set wherein the generator operates at a high frequency and the high frequency energy is converted to a relatively low frequency.
It is another object of the present invention to provide an improved prime mover and generator set wherein the output of the generator is converted to an average current having a waveform substantially sinusoidal in shape.
It is still a further object of the present invention to provide an improved energy conversion system which utilizes a multiphase, high frequency generator source for generating single phase, low frequency electrical energy.
It is another object of the present invention to provide an improved energy conversion system which converts multiphase, high frequency alternating current energy to single phase, low frequency alternating current energy wherein the power output of the system is a time modulated waveform having a sinusoidal characteristic.
It is another object of the present invention to provide a system of the above described type including arc suppressor circuit means for protecting the switching devices from arcing.
It is another object to provide a system of the above described type providing means for protecting the alternator system from high output voltages resulting from improper loading or the like.
It is still another object to provide a system of the above described type providing means for protecting the alternator from load faults.